sfc: falcon: remove duplicated bit-wise or of LOOPBACK_SGMII

Bit pattern LOOPBACK_SGMII is being bit-wise or'd twice; remove the
redundant 2nd LOOPBACK_SGMII

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>